Abstract: Errors in reading data with a magnetic head from reference zones on tracks t differing radii of a magnetic memory disc rotating at constant angular velocity are determined. Each track includes at least one reference zone including a plurality of initial cells. Each initial cell has magnetic flux variations which when read by the head result in a first waveform that includes undulations enabling the head to be properly servo controlled to a position over the center of the track and a further undulation that is distinguishable from the head positioning undulations because it has a larger amplitude. The initial cells are followed by plural binary bit representing data cells each having magnetic flux variations which when read by the head result in a second waveform including a data representing undulation that has approximately the same amplitude as the further undulations. The undulations are susceptible of having varying amplitudes as the head is positioned at different positions of the disc.